Preheat oven to about 170 C / Gas 4 and line a large cake tin.
Mix the butter, sugar and egg until smooth. Add the bananas and stir until smooth. Add the cocoa and ground hazelnuts. Mix well. Sift in the flour, baking powder and bicarbonate then add the milk and stir until combined. Pour the mixture into the prepared tin.
Bake for about an hour or so, until a skewer inserted into the centre comes out clean.
*Optional - 70% cocoa good quality dark chocolate cut into little chunks or chocolate chips.